The present invention generally relates to Q-switched lasers. More particularly, the present invention relates to so-called microchip lasers, which are passively Q-switched by means of spinel type saturable absorbers.
Passive Q-switching of lasers by bleaching of optical filters is almost as old as the laser itself. However, until recently, there were no bleachable filters (i.e. saturable absorbers) suitable for lasers emitting light of wavelengths longer than about 1.1 xcexcm. During the last years, a number of bleachable materials that could be candidates for saturable absorbers in lasers emitting light of wavelengths longer than 1.1 xcexcm have been suggested. However, up to date, all available materials suffer from drawbacks that prevent reliable operation and/or manufacture of such lasers.
For example, semiconductor saturable absorber mirrors (SESAMs) have been proposed as Q-switches. However, these devices have a high susceptibility to optical damage, and therefor limit the available output power of the lasers.
In addition, cobalt-doped dielectric crystals have been proposed. The dielectric host materials proposed include garnets, LMA and ZnSe. These cobalt-doped crystals have an absorption band at the appropriate wavelength. These materials, however, suffer from various drawbacks.
In garnets, the excited state (i.e. the bleached state) lifetime of cobalt ions is much shorter than the duration of a typical Q-switched laser pulse. Therefore, the power build-up will be limited due to the short lifetime of the bleached state.
LMA is a uniaxial crystal, and the absorption cross-section of the cobalt ions in this host depends on the polarisation of the bleaching light. Only in one polarisation direction is the absorption cross-section of the cobalt ions sufficiently large for achieving Q-switching without focusing the laser light onto the bleachable filter.
Also, cobalt-doped ZnSe can be used as a bleachable filter. However, reliable manufacture of this material has failed, leading to poor performance of Q-switched lasers based thereupon.
It is an object of the present invention to provide a reliable Q-switched microchip laser arrangement. In particular, it is an object of the present invention to provide a Q-switched laser that can be optically pumped by means of a laser diode in a longitudinal configuration.
One advantage of the present invention is that Q-switched operation is achieved for wavelengths around 1.5 xcexcm, which is an eye-safe wavelength. Therefore, the Q-switched laser according to the present invention is suitable for range-finding in urban environments.
Another advantage is that the laser arrangement according to the present invention can be actuated by means of a standard flashlight battery. This fact allows the laser to be portable and hand-held.
These objects and advantages are obtained by a laser arrangement according to the appended claims.
A laser arrangement according to the present invention comprises a first chip of active material operative to emit radiation in the near infrared spectral region; a second chip of optically bleachable material, which can be bleached by radiation in the near infrared spectral region; a pump diode for exciting the active material; two mirrors enclosing both said first chip and said second chip, in order to form a resonant laser cavity; wherein the optically bleachable material comprises a spinel type crystal doped with cobalt ions.
In this specification, the expressions xe2x80x9coptically bleachable materialxe2x80x9d and xe2x80x9csaturable absorberxe2x80x9d are to be regarded as two different names for the same material. Namely, a material that can achieve an increased transmittance when bleached (saturated) by light, compared to its unbleached (unsaturated) state.
Also, by pumping in a longitudinal configuration, it is meant that the pump light is incident into the active laser material in a direction that is essentially co-linear with the propagation direction of laser light through the active material. Often, this configuration is referred to as longitudinal pumping.
In a preferred embodiment, the active material is Er-glass. Preferably, the Er-glass material also comprises ytterbium ions that promote absorption of pump light and energy transfer to the erbium ions.
The chip of active material can be bonded to the chip of bleachable material, to form a monolithic body. Bonding the two chips together reduces losses in the laser cavity and makes the arrangement smaller in size. Also, the cavity mirrors are preferably dielectric stacks that are deposited directly onto the first chip and the second chip. The present invention allows bonding of the chips to a monolithic body, since no focusing is needed inside the laser cavity for Q-switching to be achieved.
It has been found that when spinel type crystals are made very thin, they tend to bend. In order to prevent this from happening, the chip of spinel type bleachable absorber must be made sufficiently thick. Typically, a thickness of at least 0.1 mm is needed. In order for the small signal (i.e. non-bleached) transmission through the absorber to be high enough for bleaching to occur at such thickness of the absorber, the concentration of cobalt ions in the spinel host must be reduced. Typically, the absorption coefficient of said bleachable absorber is lower than about 1 cmxe2x88x921.
In order to reduce the thermal load on the active material, pump light is preferably launched into the active material through the absorber bonded thereto. When pump light is launched into the active material, the major part of this pump light is absorbed near the surface of the material. By having the material bonded to another material, in this case the absorber, heat is transferred thereto and a cooling effect is achieved.
Suitable materials for the bleachable absorber according to the present invention are a) zinc-aluminum spinel (ZnAl2O4), b) zinc-gallium spinel (ZnGa2O4), c) lithium-gallium spinel (LiGa5O8), and d) magnesium-aluminum spinel (MgAl2O4). All of these materials are suitable for Q-switching of Er-lasers, the latter of which will be explained in detail below.